The present invention concerns the improved process for the preparation of 2,3,4,5-tetrachloro-6-(trichloromethyl)pyridine, hereinafter referred to as heptachloropicoline.
Heptachloropicoline is a known compound having been previously prepared by a number of processes. This compound has uses as a pesticide; and is also employed as a chemical intermediate in the preparation of other highly desired pesticide products. Previous methods for preparing thiscompound include those described in the following patents as well as the prior art noted therein. U.S. Pat. No. 3,256,167; 3,420,833; 3,732,230; 4,227,001; and 4,256,894.
Thus, U.S. Pat. No. 3,256,167 describes the continuous liquid-phase reaction of alpha-picoline hydrochloride with chlorine preferably in the presence of a Lewis catalyst or under UV radiation. However, this process requires a large excess of chlorine and a long reaction time.
U.S. Pat. No. 3,420,833 describes a process for preparing polychlorinated aromatic heterocyclic nitrogen compounds whereby alpha-picoline is reacted in the gas phase with gaseous chlorine at a temperature of from 400.degree. C. to 700.degree. C. A large excess of chlorine is required; but heptachloropicoline is neither mentioned nor exemplified.
U.S. Pat. No. 3,732,230 describes the reaction of liquid alpha-picoline hydrochloride with chlorine under a pressure of hydrogen chloride. However, this reaction must be run under pressure as well as requiring a reaction time of from 6 to 18 hours.
U.S. Pat. No. 4,227,001 describes the liquid phase reaction of 2,4-dichloro-6-(trichloromethyl) pyridine with chlorine at a temperature up to 220.degree. C. in the presence of a Lewis acid catalyst and at superatmospheric pressure. Here too, reaction times of about eight hours are required.
U.S. Pat. No. 4,256,894 describes the reaction of a chloro-substituted 6-(trichloromethyl)pyridine in the liquid state with chlorine in the presence of a Lewis acid catalyst. However, here too long reaction times are required, and the heptachloropicoline is obtained in very low yield in a mixture of other chlorinated pyridines.